1. Field of the Invention
The present invention relates to a process for producing a polymer or copolymer to be appropriately used for tire treads having excellent abrasion resistance, high tensile strength, steering stability and low rolling resistance. More particularly, the invention relates to a process for producing a conjugated diolefin polymer or copolymer by polymerizing a conjugated diolefin or copolymerizing a conjugated diolefin and an aromatic vinyl compound by using a specific initiator in a relatively short time and effectively modifying polymerization- or copolymerization-active terminals with a tin compound, a silicon compound and/or an organic compound containing a hetero atom, said specific initiator being a reaction product between a secondary amine compound and an organic lithium compound and/or a lithium amide compound, a reaction product between a specific alcohol and an organic lithium compound and/or a specific lithium alkoxide compound. The invention also relates to a rubber composition containing such a polymer or copolymer and suitable for use in tire treads.
2. Related Art Statement
With increased demand for low fuel consumption in automobiles, conjugated diolefin polymers or copolymers having all of excellent abrasion resistance, low rolling resistance, steering stability and high tensile strength have been recently requested as rubber materials, particularly, for tire treads.
In order to reduce rolling resistance of the tire, an energy loss of a vulcanized rubber has to be decreased. As indices for evaluating the vulcanized rubber, rebound resilience at 50.degree. C.-80.degree. C., tan .delta. at 50.degree. C.-80.degree. C. Goodrich heat generation, etc have been employed. Raw rubber materials having larger rebound resilience at 50.degree.-80.degree. C., smaller tan .delta. at 50.degree. C.-80.degree. C. or smaller Goodrich heat generation are preferable.
Up to now, styrene-butadiene copolymers having various structures polymerized with organic lithium initiators and styrene-butadiene copolymers having terminals of copolymers modified with functional groups have been proposed as raw rubber materials having large rebound resilience. For example, there are styrene-butadiene copolymers having their terminals modified or coupled with tin compounds as well as styrene-butadiene copolymers having their terminals modified with a nitrogen-containing compound such as an isocyanate compound, an oxazolidinone compound, or a dialkylaminobenzophenone compound.
However, the polymer or copolymer obtained by using the organic lithium initiator has a smaller amount of functional groups introduced therein through modification of the terminals of the polymer or copolymer. Accordingly, it was difficult to realize the demand for improving the above physical properties such as abrasion resistance and reduced rolling resistance.
Formerly, two processes were known to increase the amount of functional groups introduced. That is, the amount of functional groups introduced is increased (1) by effecting the polymerization with use of a lithium amide initiator and modifying or coupling the terminals of the resulting polymer or copolymer (Japanese patent application Laid-open No. 59-38,209 and Japanese patent publication No. 5-1,298), or (2) by preliminarily reacting an organic lithium initiator with a vinyl compound having functional groups in a small scale, and effecting the polymerization with use of the resulting reaction product, while the terminals of the polymer or copolymer are modified (Japanese patent publication No. 5-1298). However, it is reported in these publications that a longer time is required to obtain the polymer or copolymer as compared with a case where polymerization is effected with the organic lithium initiator alone. Further, reactivity of the terminals of the polymer or copolymer drops, so that the efficiency of a modifying reaction is unfavorably low. Thus, it has been difficult to increase the introduced amount of functional groups.
On the other hand, in a case that a diolefin or an olefin which is substituted with a polar group such as an ester, a nitrile or an aryl group is polymerized (copolymerized) with an alkaline metal amide compound, it is known that a yield of the polymer or copolymer is increased through the addition of alkaline metal alkoxide (Japanese patent application Laid-open No. 54-100,484).
However, since this process also needs a long time to obtain the polymer or copolymer in the case of the polymerization of the conjugated diolefin and/or aromatic vinyl compound, the above-mentioned problem of the prior art could not be solved by this process, either.
That is, in actuality, the speed of the polymerization of the conjugated diolefin and/or the aromatic vinyl compound with use of the lithium amide initiator is small, and the reactivity of active terminals of the polymer or copolymer is small.
Tires are broadly classified into (1) low fuel consumption tires making much of low rolling resistance, (2) large size tires to be used in trucks or buses making much of abrasion resistance, and (3) high performance tires making much of steering stability and high tensile strength at high speeds, depending upon uses thereof.
As to low fuel consumption tires, the tires are improved to some extent by the above technical idea. As to the high performance tires, trials have been made to increase an amount of an extender oil or to add a special liquid rubber or the like. However, these trials both deteriorate the rolling resistance and abrasion resistance, so that it is difficult to simultaneously satisfy all of the performances mentioned above.
For large tires, although high cis-polybutadiene has been conventionally mainly used, it is unfavorably difficult to modify or couple the terminals of the polymer.
That is, it is an actual situation that a conjugated diolefin polymer or copolymer applicable to all of the above uses has not been discovered.